The invention relates generally to modulating cell adhesion and to inhibiting the binding of fibrinogen and other proteins to blood platelets, and inhibiting the aggregation of blood platelets specifically to the IIb/IIIa fibrinogen receptor site. Fibrinogen is a glycoprotein, present in blood plasma, which participates in platelet aggregation and in fibrin formation. Platelets are cell-like anucleated fragments, found in the blood of all mammals, which participate in blood coagulation. Interaction of fibrinogen with the IIb/IIIa receptor site is known to be essential for normal platelet function.
When a blood vessel is damaged, platelets adhere to the disrupted subendothelial surface. The adherent platelets subsequently release biologically active constituents and aggregate. Aggregation is initiated by the binding of agonists, such as thrombin, epinephrine, or ADP to specific platelet membrane receptors. Stimulation by agonists results in exposure of latent fibrinogen receptors on the platelet surface, and binding of fibrinogen to the glycoprotein IIb/IIIa complex.
Attempts have been made to use natural products and synthetic peptides to study the mechanism of platelet aggregation and adhesion.
Rouslahti and Pierschbacher, Science, 1987, 238, pp. 491-497, describe adhesive proteins such as fibronectin, vitronectin, osteopontin, collagens, thrombospondin, fibrinogen, and von Willebrand factor present in extracellular matrices and in the blood. The proteins contain the tripeptide arginine-glycine-aspartic acid as their cell recognition site. The tripeptides are recognized by at least one member of a family of structurally related receptors, integrins, which are heterodimeric proteins with two membrane-spanning subunits. The authors state that the conformation of the tripeptide sequence in the individual proteins may be critical to recognition specificity.
Cheresh, Proc. Nat'l. Acad. Sci. U.S.A. 1987, 84, pp. 6471-6475, describes an Arg-Gly-Asp directed adhesion receptor expressed by human endothelial cells that is structurally similar to the IIb/IIIa complex on platelets but antigenically and functionally distinct. The receptor is directly involved in endothelial cell attachment to fibrinogen, von Willebrand factor, and vitronectin.
Pierschbacher and Rouslahti, J. of Biol. Chem., 1987, 262, 36, pp. 17294-17298 describe stereochemical influence of the sequence Arg-Gly-Asp-Xaa, where Xaa is one of the 20 natural L-amino acids other than Met, Cys, His, Trp or Gly on binding specificity of peptides containing the tripeptide sequence Arg Gly Asp. The authors showed that cyclization of the sequence Gly-Pen-Gly Arg-Gly-Asp-Ser-Pro Cys-Ala (where Pen is penicillamine), by forming a disulfide bridge between Pen and Cys, rendered the peptide ineffective at inhibiting attachment to fibronectin. In Proc. Nat'l. Acad. Sci. U.S.A., 1984, 81, pp. 5985-5988, the same authors describe variants of the cell recognition site of fibronectin that retain attachment-promoting activity. The tetrapeptide Arg-Gly Asp-Ser is described as the minimal structure recognized by cells in the large, adhesive glycoprotein fibronectin. Peptides having portions Arg Gly Asp Ser- are described in U.S. Pat. Nos. 4,589,881 and 4,614,517. Peptides having portions -Arg-Gly-Asp-R wherein R is selected from Thr or Cys or other amino acid having the same cell-attachment activity as fibronectin, are described in U.S. Pat. No. 4,578,079.
Ruggeri et al., Proc. Nat'l. Acad. Sci. U.S.A., 1986, 83, pp. 5708-5712, describes a series of synthetic peptides, designed in lengths to 16 residues, that contain the sequence Arg-Gly Asp-Val, which inhibit fibrinogen binding to platelets.
While it is known that the tripeptide sequence Arg-Gly-Asp is present in certain polypeptides which can duplicate or inhibit the cell attachment-promoting effects of fibronectin and vitronectin, the tripeptide Arg Gly-Asp has low activity. There is little understanding of the influence on binding specificity of other amino acids in the polypeptide. Applicants have prepared small cyclic heptapeptides which contain the tripeptide sequence Arg Gly Asp which are platelet aggregation inhibitors.